1. Field of the Invention
This invention relates to the production of a nitration grade toluene product. More particularly, this invention relates to the production of a nitration grade toluene product from the process of thermal cracking hydrocarbons.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In an olefin production plant, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated.
This mixture, after preheating, is subjected to hydrocarbon thermal cracking using elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker). This thermal cracking is carried out without the aid of any catalyst.
The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and molecular hydrogen (hydrogen).
This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, and fuel oil. After the separation of these individual streams, the remaining pyrolysis gasoline contains essentially C4 hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product.
The C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (both cis and trans isomers).
The C5 and heavier stream contains primarily (in major proportion, greater than 50% by weight) C6 through C8 hydrocarbons, both aromatic and non-aromatic (alkanes and alkenes). Aromatics are separated from this stream by way of a solvent extraction process, and the aromatics in the extract are themselves separated to recover individual plant product streams such as a benzene product, a toluene product, and the like. The non-aromatics are typically passed to the gasoline pool.
There are several marketable grades of toluene product that can be recovered from the aromatic rich extract that is recovered from the solvent extraction process. The grades of toluene product are “commercial” which contains at least 95 weight percent (wt %) toluene, “nitration grade” which contains at least 98.5 wt % toluene, and “TDI grade” which contains at least 99.9 wt % toluene, all wt % based on the total weight of the product.
This invention is directed toward producing an optimum amount of nitration grade toluene product from the aromatic (primarily benzene and toluene) rich extract of a solvent extraction.